1. The Field of the Invention
The present invention relates to systems and methods for monitoring traffic. More particularly, the present invention relates to systems and methods for measuring the speed of a vehicle and more specifically to systems and methods for measuring the speed of a vehicle using different sensor configurations.
2. Background and Relevant Art
As the number of vehicles on roadways increases and traffic becomes more congested, monitoring existing traffic and planning for future traffic become more important. Monitoring traffic in particular has received increasing attention from traffic planners. The information gathered from monitoring various aspects of traffic is useful for a variety of different purposes including law enforcement, traffic planning, safety, incident management, and traffic reporting.
The speed of vehicles, for example, traveling on a roadway is one of the activities that is often monitored. However, monitoring the speed of vehicles using people is difficult and expensive. As a result, traffic sensors that automatically monitor and measure the speed of vehicles are becoming increasingly common on roadways. Speed measurements are typically taken from two types of sensor configurations that are referred to as a side fire sensor configuration and a forward fire sensor configuration. As their names suggest, a side fire sensor configuration measures the speed of the vehicle as the vehicle passes by the measuring sensor. In a forward fire sensor configuration, the vehicle is either traveling towards or away from the measuring sensor. Current side fire systems and forward fire systems, however, have various disadvantages.
For example, one side fire method for measuring the speed of a vehicle is based on measuring a detection time that is related to a specific detection zone. As the speed of a vehicle increases, the detection time for the detection zone decreases. This method, however, assumes that every vehicle has an average length. As a result, the speed measurement for vehicles that do not have an average length is skewed. In addition, the sensor used in this side fire configuration must be calibrated to remove the length of the detection zone from the speed computation.
Another side fire method for measuring the speed of a vehicle is based on a small detection zone that is located within a larger detection zone. In this method, the elapsed time between an initial detection of the vehicle in the larger zone and an initial detection of the vehicle in the smaller zone is combined with the distance between the edges of the two zones to determine the speed of the vehicle. Because the zones are different, the signals used to detect the vehicles are dissimilar. The dissimilarities between the responses of the two signals in the two zones introduces variability into the speed measurement.
Another side fire method for measuring the speed of a vehicle uses two infrared beams that are separated by a lateral distance. The elapsed time between the interruption of each beam and the lateral distance between the two beams are used to calculate the speed of a vehicle. This method, however, requires two relatively narrow infrared beams for each lane of traffic that is being observed. The hardware required to implement this method can introduce significant cost.
One forward fire method for determining the speed of a vehicle is based on distance measurements using a laser. Several distance measurements are performed in a short period of time and the change in the target distance is calculated. This method can potentially result in erroneous results if the laser is swept along the side of the vehicle as the distance measurements are taken.
Another forward fire method is a continuous wave Doppler radar that determines the speed of a vehicle based on the Doppler shift. However, devices relying on the Doppler shift cannot distinguish between vehicles and cannot determine the range to a particular vehicle. Other forward fire methods define a pair of detection zones. The distance between the two zones and the elapsed time between detections in the two zones is used to compute the speed of a vehicle. These methods are often inaccurate as the signals returned by the respective zones are often dissimilar as previously mentioned.
The speed of a vehicle can also be measured by embedding inductive loops in a roadway. The signals generated by a pair of loops are processed to extract a time delay. The time delay and the distance between the two loops can be used to determine a vehicle's speed. The primary disadvantage of this method is that the inductive loops are difficult and expensive to install and maintain. In addition to being costly, installing and maintaining the inductive loops typically interferes with traffic.